GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire)
The GLIMPSE Spring '05 delivery contains a "Catalog" and an "Archive", both of which are served through Gator. The 'Catalog' is a highly reliable source list (>99.5%), while the 'Archive' is a more complete source list but less reliable (>98%). The 'Archive' can be used scientifically with judicial use of source quality flags, and other information (signal-to-noise, flux levels, etc.). delivery document for more information regarding these two source lists.
Catalog and Archive Fields and FlagsEach entry in the GLIMPSE Catalog and Archive has the
following information:
designation SSTGLMC GLLL.llll+-BB.bbbb, SSTGLMA GLLL.llll+-BB.bbbb position l, b, dl (error), db (error), ra, dec, dra (error), sdec (error) flux magi, dmagi (error), Fi, dFi (error), Fi_rms diagnostic skyi, SNi, dens_i, # detections Mi out of Ni possible flags confusion flag, source quality flag (i), flux calculation method flag (i) where i is the IRAC wavelength number (IRAC bands 1-4): 3.6 um, 4.5 um, 5.8 um and 8.0 um.Details of the fields are as follows:
The 'designation' entry can have the following set of information:
SSTGLMC GLLL.llll+-BB.bbbb, SSTGLMA GLLL.llll+-BB.bbbb
This is the object designation or ``name" as specified by the IAU recommendations on source nomenclature. It is derived from the coordinates of the source, where G denotes Galactic coordinates, LLL.llll is the degrees of Galactic longitude, and +-BB.bbbb is the degrees of Galactic latitude. The coordinates are preceded by the acronym SSTGLMC (Spitzer Space Telescope GLIMPSE Catalog) or SSTGLMA (Spitzer Space Telescope GLIMPSE Archive).
There are 8 'position' related fields:
l, b, dl (error), db (error), ra, dec, dra (error), ddec (error)
The position is given in both Galactic (l,b) and equatorial (RA, Declination) J2000 coordinates, along with estimated uncertainties. The pointing accuracy is 1 arcsec (Werner et al. 2004). The SSC pipeline does pointing refinement of the images based on comparison with the 2MASS Point Source Catalog, whose absolute accuracy is typically <0.2 arcsec (Cutri & Skrutskie 2000). After applying the SSC geometric distortion corrections and updating to the 2MASS positions, the GLIMPSE point source accuracy is typically 0.3 arcsec absolute accuracy, limited by undersampling of the point-spread function. The position uncertainties are calculated by the bandmerger based on the uncertainties of individual detections, propagated through the calculation of the weighted mean position.
There are 20 'flux' related fields:
magi, dmagi (error), Fi, dFi (error), rms_Fi
For each IRAC band (i = 3_6, 4_5, 5_8 and 8_0) the fluxes are expressed in magnitudes (magi) and in mJy, Fi. Each flux is the error-weighted average of all independent detections of a source. The zeropoints for converting from flux to magnitude are from Martin Cohen for the IRAC bands and Cohen et al. 2003 for 2MASS and given in Table 2.
Table 2. Zeropoints for Flux to Magnitude Conversion Band J H K [3.6] [4.5] [5.8] [8.0] Zeropoints (Jy) 1594 1024 666.7 277.5 179.5 116.5 63.13
The flux/magnitude uncertainties (Fi_err; magi_err) are computed during the photometry stage and take into account photon noise, readnoise, goodness of flat fielding, and PSF fitting (Stetson 1987). Magnitude uncertainties are estimated to be <0.2 mag for the Catalog and <0.3 mag for the Archive. The uncertainties are smaller in Bands 1 and 2 than Bands 3 and 4 due to lower backgrounds.
The rms deviation (Fi_rms) of the individual detections from the final flux of each source is provided.
To figure out what condition caused the null value in the flux column you would need to look at the M & N values and the source quality flag. If the object wasn't detected in that band, but there was IRAC coverage, then you would see that in the M & N values (number of actual detection = M, number of possible detections =N). If there was not IRAC coverage, that would be seen in the N value (N=0). Fluxes can also be nulled in the Catalog if they are brighter than our bright limits or within the wings of a saturated star. The bright limit flux is 439 mJy (7 mag), 450 mJy (6.5 mag), 2930 mJy (4 mag) and 1590 mJy (4 mag) for the four bands, to remove sources at nonlinear response levels.
There are 20 'flux diagnostics' related fields:
sky_i, sn_i, dens_i, # detections Mi out of Ni possible
The associated flux diagnostics are a local background level (skyi) (i = 1,4) in MJy/sr, a Signal/Noise (SNi), a local source density (srcdensi) (number of sources per square arcmin), and number of times (Mi) a source was detected out of a possible number (Ni). The local background, an output of DAOPHOT, is provided because high backgrounds were shown to affect the reliability of IRAS sources, and for IRAC as well (especially Bands 3 and 4). However, the effects may not be easily characterizable in the quoted error. The Signal/Noise is the flux (Fi) divided by the flux error (Fi_err). The local source density is measured as follows: The individual IRAC frame is divided into a 3 times 3 grid, each section being 1.71 arcmin x 1.71 arcmin. A source density is calculated for each section (number of sources per arcmin^2), and is assigned to each source in that section. The local source density can be used to assess the confusion in a given region, along with the internal reliability. Mi and Ni can be used to estimate reliability. Ni is calculated based on the areal coverage of each observed frame; due to overlaps some areas are observed more than twice per band.
There are 5 'flag' related fields:
confusion flag, source quality flag (i), flux calculation method flag (i)
There are three types of flags: the confusion flag, the source quality flag and the flux calculation method flag. The confusion flag will be set for sources that are within 1 arcsec of each other. The source quality flag provides a measure of the quality of the point source extraction. The flux calculation method flag describes how the final Catalog/Archive flux was determined.
The Confusion Flag will be set when another source is within one arcsecond of the current source. Preliminary results show approximately 0.25% of the sources in the Archive will have this flag set. Note: the confusion flag is not yet set in the April 12, 2005 data release.
The Source Quality Flag (SQF) is generated from SSC-provided masks and the GLIMPSE post-processing pipeline, after point source extraction on individual IRAC frames. Each source quality flag is a binary number allowing combinations of flags (bits) in the same number. Flags are set if an artifact (e.g., a hot or dead pixel) occurs near the core of a source - i.e. within about 3 pixels. A non-zero SQF will in most cases decrease the reliability of the source. Some of the bits, such as the DAOPHOT tweaks, will not compromise the source's reliability, but will likely increase the error assigned to the source flux. Since the current data release does not include a confusion flag, users may wish to make use of two bits in the source quality flag, bits 13 and 14, set during the bandmerge step of data processing. These are set when the bandmerger has more than one possible candidate to merge with a given source. Bit 13 is set during the merge within a given band, and bit 14 is set during merges across bands. About 20% of the sources have bit 14 (cross-band) set, and less than 1% have bit 13 (in-band) set.
Table 3 shows the SQF sequence for the April 12, 2005 data release. Some of the flags have not been implemented, but will be in future data releases. For example, SSC and the GLIMPSE team are working on an algorithm for stray light masking. We have determined that false sources from these regions do not make it into the Catalog due to our 2+1 source selection criterion. In addition, our photometry algorithm has been modified substantially to find sources in high background regions that gives it the ability to find sources in stray light and banded regions as well, increasing the photometric uncertainties accordingly.
The value of the SQF is sum{2^{(bit-1)}}. For example, bit 1 corresponds to 2^0 = 1. If the SQF is 0, the source has no detected problems. More information about these flags can be found in the Spitzer Observer's Manual (SOM) and the GLIMPSE Pipeline Description (GPD).
Source Quality Flag (SQF) Bits
SQF bit Description Ref Source Status
1 poor pixels in dark current SOM SSC pmask OK
2 flat field questionable SOM SSC dmask not working a
3 latent image SOM SSC dmask OK b
7 recorrected muxbleed GPD SSC,GLIMPSE not done c
8 hot, dead or otherwise unacceptable pixel SOM SSC pmask OK
9 MUX bleed uncertain or incomplete SOM SSC dmask not working a
10 DAOPHOT tweak positive GPD GLIMPSE OK
11 DAOPHOT tweak negative GPD GLIMPSE OK
13 confusion in in-band merge GPD GLIMPSE OK
14 confusion in cross-band merge GPD GLIMPSE OK
15 column pulldown corrected GPD GLIMPSE OK
16 banding corrected GPD GLIMPSE OK
17 stray light SOM SSC,GLIMPSE not done c
19 nonlinear correction not applied SOM SSC dmask not sure
20 saturated star wing region GPD GLIMPSE OK
30 within three pixels of edge of frame GPD GLIMPSE OK
a According to SSC.
b Due to the high sky backgrounds in the GLIMPSE fields, we have not seen
evidence for latent sources in the images, even though they are flagged.
c We plan to have these implemented in a future data release.
Flux calculation Method Flag (MF).
This is a flag for each band indicating how the final averaged flux was determined by weighting individual flux measurements. The procedure is especially important when the individual measurements have different SQFs or significantly discrepant values.
Table 4.
Method Flag (MF) MF Description 0 all fluxes in agreement within errors 1 some discrepant fluxes discarded 2 all fluxes were discrepant; use this source with caution
REFERENCES
Benjamin, R.A., Churchwell, E., Babler, B., Bania, T.M., Clemens, D.P., Cohen, M., Dickey, J.M., Indebetouw, R., Jackson, J.M., Kobulnicky, H.A., Lazarian, A., Marston, A.P., Mathis, J., Meade, M.R., Seager, S., Stolovy, S.R., Watson, C., Whitney, B.A., Wolff, M.J., and Wolfire, M.G. 2003, PASP, 115, 953 Cohen, M., Wheaton, W.A., and Megeath, S.T. 2003, AJ, 126, 1090. Cutri, R., & Skrutskie, M. 2000, www.ipac.caltech.edu/2mass/releases/second/doc/sec1_66.html. Fazio et al., 2004, ApJS, 154, 10. Stetson, P. 1987, PASP, 99, 191. Werner et al. 2004, ApJS, 154, 1.